Upload drop metadata to IPFS

Uploads every saved item's media and metadata to IPFS, then saves the resulting folder as the drop's draft base URI. Run it after POST /api/v2/drops/{slug}/items/media/save-batch; the drop needs at least one item. It does not change the contract: call POST /api/v2/drops/{slug}/publish once it completes.

This is the reveal. The base URI it writes points at the real item metadata, so a drop published after it has completed is revealed as soon as it is live. To reveal later, publish without running it (showing the prereveal item from POST /api/v2/drops/{slug}/prereveal-item), then run it and publish again when ready.

Only ERC-721 SeaDrop V1 drops are supported; any other drop type returns 400. An ERC-1155 self-mint drop uploads each item's metadata when the item is saved or updated.

Only one upload runs per drop at a time. Calling again while one is running returns that upload's id. Poll GET /api/v2/drops/{slug}/metadata/ipfs/{workflow_execution_id} until status is completed or failed.